1. Field of the Invention
The present disclosure relates to a backlight unit for flat panel display device. Specifically, the present disclosure relates to a backlight unit having a light emitting device (or, LED) for enhancing turn-on property of a fluorescent light source.
2. Discussion of the Related Art
Nowadays, due to the characteristics of light weight, slim thickness, low consumption electric power, the liquid crystal display device (or, LCD) is being widely applied. The LCD is applied to a portable computer such as note book PC, official automation devices, a audio/video devices, etc. The most common LCD shows the picture data by modulating the luminescence of the light incident from the backlight unit by controlling the electric field applied to the liquid crystal layer.
As the liquid crystal display device is not a self-luminescent element, the liquid crystal display device requires a backlight unit for irradiating light to the liquid crystal display panel. There are two kinds of the backlight units for LCD, one is the direct type backlight unit and the other is the edge type backlight system. For the edge type backlight unit, light source such as a fluorescent light source is equipped at the circumferences of the transparent light guide panel. The light radiated from the fluorescent light source to the side surface of the light guide panel is refracted and/or reflected to the front side on which the LCD panel is disposed. On the other hands, for the direct type backlight unit, a plurality of fluorescent light sources are disposed under the back side of the LCD panel so that the light is directly radiated from the light source to the overall surface of the LCD panel. As more light sources are used, the direct type has advantages for making a wide area LCD panel easily and for enhancing the brightness effectively.
FIG. 1 illustrates a liquid crystal display device adapting a direct type backlight unit according to the related art. Referring to FIG. 1, a liquid crystal display device according to the related art comprises a panel guide PG, a liquid crystal display panel LCDPC disposed into the panel guide PG, a cover bottom CBC, a backlight unit BLUC disposed into the cover bottom CBC and a top case TC for fastening the liquid crystal display panel LCDPC.
As the panel guide PG is made by the molding process, the inside wall of the panel guide PG has stepped profile. Due to the stepped side, the liquid crystal display panel LCDPC can be mounted inside of the panel guide PG.
The liquid crystal display panel LCDPC comprises a color filter array panel and a thin film transistor (or, TFT) array panel. Between the color filter array panel and the TFT array panel, a plurality of liquid crystal cells are arrayed in active matrix type. At each liquid crystal cell, a thin film transistor is provided for applying a video signal. As the refraction ratio of the liquid crystal cell is changed according to the video signal, the picture and/or video corresponding to the video signal is represented on the liquid crystal display panel LCDPC. On the TFT array panel of the liquid crystal display panel LCDPC, attached are a gate tape carrier package in which a plurality of gate IC for generating the gate signals and a data tape carrier package in which a plurality of data IC for generating the data signals.
The top case TC is made as a closed rectangular rim shape having a horizontal part and a vertical part, which are bent perpendicularly each other. The top case TC surrounds the circumferences of the liquid crystal display panel LCDPC and the panel guide PG.
The cover bottom CBC comprises a bottom surface and four side surfaces vertically surrounding the bottom surface. That is, the cover bottom CBC has opened top portion. At outside of any side surfaces, there may be a support side surrounding the circumference of the cover bottom CBC.
The backlight unit BLUC comprises a plurality of light source LSC generating light, a reflection sheet REFC disposed under the light sources LSC, a diffusion plate DIFC covering the light sources LSC, and optical sheets OPTC disposed on the diffusion plate DIFC.
Each of the plurality of light source LSC is, for example, an external electrode fluorescent lamp (EFFL) comprising a glass tube, an inertia gas filling up the glass tube, and anode and cathode electrodes built at each ends of the glass tube, respectively. The inertia gas is filled with the glass tube and a fluorescent material is coated at the inner surface of the glass tube. The plurality of light sources LSC are held by lamp holders LH mounted on the bottom surface of the cover bottom CBC.
The diffusion plate DIFC diffuses the light radiated from the plurality of the light source LSC to the liquid crystal display panel LCDPC with widen incident angle. The diffusion plate DIFC can be made of polymethylethacrylate (PMMA) material or glass for preventing deformation by heat from the light source LSC.
In this structure, the plurality of the light sources LSC are housed in a dark space so that the light sources LSC can be left under dark conditions for a long time. Specifically, when the light sources LSC are fluorescent lamps, as they are left under dark conditions for a long time, the existing probability of electrons in the lamps can be reduced remarkably. At this situation, if a power for the lamps is turned on, the time interval required for forming plasma in the fluorescent lamps may be longer than normal condition. As a result, a turn-on problem may be occurred in which the fluorescent lamps are turned on after the liquid crystal display device starts representing the video data.